There were some previous attempts to provide detailed documentation
of the mailserver: URI scheme, but those efforts were not successful.
Implementors interested in providing instructions for generating an
email [RFC5322] message can instead use the mailto: URI scheme
[RFC6068]. Implementors interested in referencing a message or a set
of messages available from a mailstore over IMAP [RFC3501], POP
[RFC1939], or web [RFC2616] can instead use the imap: [RFC5092], pop:
[RFC2384] or http: [RFC2616] URIs, respectively.

It should say:

There were some previous attempts to provide detailed documentation
of the 'mailserver' URI scheme, but those efforts were not successful.
Implementors interested in providing instructions for generating an
email [RFC5322] message can instead use the 'mailto' URI scheme
[RFC6068]. Implementors interested in referencing a message or a set
of messages available from a mailstore over IMAP [RFC3501], POP
[RFC1939], or web [RFC2616] can instead use the 'imap' [RFC5092], 'pop'
[RFC2384] or 'http' [RFC2616] URIs, respectively.

Notes:

This is a complement to Erratum 2756 (http://www.rfc-editor.org/errata_search.php?eid=2756); see justification there.